Digital imaging devices are currently implemented in many types of electronic devices. These electronic devices may include digital still cameras, digital video cameras, mobile camera-phones, computer web-cams, and other types of devices. Semiconductor components are commonly used to capture images and digitally process the captured images. These semiconductor components may include charge coupled devices and complementary metal-oxide semiconductor (CMOS). To capture or acquire images, digital imaging devices often use a sensor array made of an arranged pattern of photodiodes. This type of sensor array is commonly known as a “pixel array.” The pixel array may also be referred to as image pick-up unit hereafter.
Digital imaging devices may encounter “flicker.” Artificial lighting that uses an alternating current (AC) source, such as fluorescent lighting, contains a brightness modulation component known as flicker. The frequency of flicker for artificial lighting is twice the frequency of an AC source. For example, a 50 Hz AC power system and a 60 Hz AC power system result in 100 Hz and 120 Hz flicker frequencies, respectively. Flicker typically arises during image capturing when an electronic device and the frequency of an AC source are not synchronized. As an example, a frame rate commonly used in the electronics industry is 30 Hz. When this frame rate is used, flicker can result from artificial lighting employing a 50 Hz AC power system.
Each pixel element typically includes a photodiode and other integrated circuitry. The photodiode generates a current according to the amount of light detected, and a sum charge is stored in an integrated capacitor. The stored charge is then converted into an output voltage, where the output voltage is proportional to the amount of charge stored in the capacitor. After that, an analog-to-digital converter (ADC) converts the output voltages of the pixel elements into digital values, and the digital values are processed into a digital image.
Thermal leakage always exists in an image pick-up unit and may be as significant as the detected flicker envelope. The thermal leakage, once converted into direct current (DC) voltage, may be represented as a DC voltage offset. Specifically, a pixel area is substantially larger than a pixel arranged for producing a series of signals which is a function of light. Thus, there may be a chance of imaging some part of the scene which contains a flickering light source. This large area pixel array is generally referred to as a superpixel.